Interferometry with pulse broadened diode laser

ABSTRACT

Various optical systems equipped with diode laser light sources are discussed in the present application. One example system includes a diode laser light source for providing a beam of radiation. The diode laser has a spectral output bandwidth when driven under equilibrium conditions. The system further includes a driver circuit to apply a pulse of drive current to the diode laser. The pulse causes a variation in the output wavelength of the diode laser during the pulse such that the spectral output bandwidth is at least two times larger than the spectral output bandwidth under the equilibrium conditions.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.17/028,663, filed Sep. 22, 2020, which is a continuation of U.S. patentapplication Ser. No. 16/669,289, filed Oct. 30, 2019, now U.S. Pat. No.10,809,050, which is a continuation of U.S. patent application Ser. No.15/759,486, filed Mar. 12, 2018, now U.S. Pat. No. 10,495,439, which inturn is a National Phase application under 35 U.S.C. § 371 ofInternational Application No. PCT/EP2016/071791, filed Sep. 15, 2016,which claims priority to U.S. Provisional Application Ser. No.62/219,872, filed Sep. 17, 2015, and U.S. Provisional Application Ser.No. 62/265,107, filed Dec. 9, 2015, all of which are hereby incorporatedby reference.

BACKGROUND

High intensity, broad bandwidth light sources are useful forinterferometric measurements such as optical coherence domainreflectometry (OCDR), optical coherence tomography (OCT), andself-interference interferometry (SII). High intensity sources aredesirable to achieve a large number of constructively interferingphotons received back from a weakly reflecting sample that can beobserved on a detector. A continuous, smooth, wide spectrum of opticalfrequencies is desirable to achieve high spatial resolution in thedirection of optical propagation.

OCDR is an interferometric imaging method that determines the scatteringprofile of a sample along the beam by detecting light reflected from asample combined with a reference beam. Each scattering profile in thedepth direction (z) is called an axial scan, or A-scan. OCT is anextension of OCDR in which cross-sectional images (B-scans), and byextension 3D volumes, are built up from many A-scans, with the OCT beammoved to a set of transverse (x and y) locations on the sample. ModernOCT systems typically collect spectrally resolved data because thisallows simultaneous measurement of a range of depths at high depthresolution, without a signal to noise penalty. This method of collectingspectrally resolved data and translating it to a depth resolvedmeasurement via a Fourier transform across the spectral dimension isreferred to as “frequency domain” or “Fourier domain” OCT (FD-OCT).

Traditional sources for FD-OCT can be described as producing a broadbandwidth light simultaneously as with superluminescent diodes (SLD) orfemtosecond Titanium Sapphire lasers; or as producing a broad bandwidthby sequentially tuning through a range of narrow bandwidths which, whenconsidered together, constitute a broad time integrated bandwidth. Thelatter type of source may be called a swept source. Examples ofswept-sources include external cavity tunable lasers (ECTL), verticalcavity surface emitting lasers (VCSEL), and sampled grating distributedBragg reflector lasers (SG-DBR) among others. Simultaneousbroad-bandwidth sources are typically detected with a spectrallydispersing element and a linear array of photodiodes in what is calledas spectral domain OCT (SD-OCT). Swept sources are typically detectedover time using one or two single element photo-detectors, thereforeencoding the spectral information in the time dimension, in what isreferred to as swept-source OCT (SS-OCT). Exposure times for SD-OCTsystems in the human eye are limited by phase washout (i.e.,cancellation of signal during a measurement period due to axial/lateralmotion) to about 100 μs. Exposure times for SS-OCT systems are limitedto about 1 ms. A hybrid swept source, spectral domain (SS/SD-OCT)arrangement has been described which allowed longer exposure times thana standard SD-OCT system without phase washout (see for example, Yun, S.H., Tearney. G., de Boer, J., & Bouma, B. (2004). Pulsed-source andswept-source spectral-domain optical coherence tomography with reducedmotion artifacts. Optics Express, 12(23), 5614-5624).

Both SD-OCT and SS-OCT systems have found use in commercialapplications, a notable example being the field of ophthalmology. Thehigh brightness simultaneous or swept broad bandwidth sources used intoday's commercial ophthalmic OCT systems account for a significantfraction of the total system cost. SS-OCT is a rapidly developing areaof OCT; however the cost remains very high, due to which widespreadcommercial acceptance has not been met yet.

Parallel OCT systems simultaneously illuminate a set of A-scans, whereastraditional point scanning OCT systems illuminate a single lateral pointat a time. Field illumination OCT is a subset of parallel OCT where theillumination is contiguous between multiple A-scans (as a line, partialfield, or full field) as opposed to spatially separated individualA-scans. Field illumination OCT offers potential benefits in terms ofcost relative to point scanning OCT, partly owing to simplifications infast beam scanning. Parallel OCT systems require similar exposure energyper unit area as traditional point-scanning OCT systems, to achievesimilar shot noise limited sensitivity. The constraints on exposureenergy and exposure time result in greater source power requirements forparallel systems compared to traditional point-scanning OCT to achievesimilar signal-to-noise ratio (SNR) in a single measurement. Some highlyparallel SD-OCT systems use two-dimensional (2D) array sensors tomeasure many points of spectra simultaneously. 2D sensors are commonlyavailable as consumer electronics such as cell phone and securitycameras, and can therefore often be found for less than the cost oflinear arrays typically found in point scanning SD-OCT systems. Theframe rates of these consumer devices are currently typically less than200 Hz. The short exposure times of SD-OCT, and the slow frame rates oflow cost 2D arrays imply a duty cycle less than 2%. This low duty cyclemeans that an attempt to construct composite scans from seriallyacquired scans illuminated with a continuous wave (CW) source will havelow efficiency (i.e., over time, more than 98% of the power from thesource will need to be blocked). Existing superluminescent diodes aremoderately expensive and insufficient in terms of power, and existingswept sources are too expensive to power low cost OCT devices that takeadvantage of the cost benefits of field illumination OCT.

Semiconductor diode lasers can easily achieve single transverse modepower levels on an order of magnitude larger than superluminescentdiodes (SLD). When a semiconductor diode material begins to lase, thespectral bandwidth narrows, usually to one or a few very narrow peakscalled longitudinal modes which are constrained by the resonance of thelaser cavity. Bandwidth as used herein refers to the width of theenvelope of the multiple longitudinal modes. Typically, consumer devicessuch as compact disc readers, laser printers, and rangefinders need thehigh intensity and spatial coherence provided by a laser, but do notrequire, or find it disadvantageous to employ a broad bandwidth such asprovided by an SLD. Although SLD and semiconductor diode lasers largelyshare the same materials, manufacturing, and packaging techniques,semiconductor diode lasers are often manufactured at low unit costsbecause of the extremely high volumes utilized in consumer electronics.

Methods of tuning and shaping the spectral output of diode lasers havebeen developed and employed for various applications including opticalcoherence tomography and related interference techniques. Thetemperature dependence of the semiconductor bandgap was one of thefirst, and most common, methods demonstrated to tune the output of adiode laser, but is usually associated with a response that is too slowand coarse for OCT. A closed loop system has been described in the artwhere a current controlled thermocouple adjusts the case temperature ofthe laser package (see for example, Bartl, J., Fíra, R., and Jacko, V.(2002). Tuning of the laser diode. Measurement Science Review volume 2section 3, hereby incorporated by reference). Rapidly tunableintracavity filters, which precisely restrict the longitudinal mode ofthe laser, are standard in swept source OCT (see for example, U.S. Pat.No. 5,949,801 hereby incorporated by reference), where it is desirableto smoothly sweep a narrow laser line across a broad bandwidth. Thecomplexity associated with this method results in a high system cost.

The total spectrum from multi-longitudinal mode lasers, whichsimultaneously produce many closely spaced, narrow bandwidth modes, isrelatively broad; however the comb structure on the spectrum causessevere sidelobe artifacts. Such multi-longitudinal mode laser spectracan be blurred over time by forcing the comb spectrum to shift slightlyduring the measurement period so that the comb peaks move to fill in thespaces in between the peaks. Non-equilibrium thermal effects and carrierdensity effects can create small changes in refractive index of thelaser cavity. These changes in refractive index effectively change theoptical length of the laser cavity such that the modes of the cavityshift slightly to blur the comb structure. Wei-Kuo Chen demonstrated thesmoothing of a comb spectrum for a depth ranging application by drivinga multimode laser with 100 picosecond long pulses which act primarily bychanging the optical length of the cavity (by carrier density effects)to shift its resonances (see for example, Wei-Kuo Chen and Pao-Lo Liu,“Short-coherence-length and high-coupling-efficiency pulsed diode laserfor fiber-optic sensors,” Opt. Lett. 13, 628-630 (1988), herebyincorporated by reference). Such short pulses are difficult to achievebecause of the impedance of the drive electronics and the packaging ofthe diode laser itself.

An interferometric imaging system closely related to OCT wasdemonstrated using a multimode diode laser and applying a sinusoidal 100Hz modulation between lasing threshold and approximately max sustainableCW current (see for example, Balboa, I., Ford, H. D., and Tatam, R. P.(2006). Low-coherence optical fibre speckle interferometry. MeasurementScience and Technology 17, 605, hereby incorporated by reference). Atthis much slower modulation frequency, thermal effects are believed todominate over carrier density effects. The comb structure is blurred tobecome more ideally Gaussian (therefore suppressing sidelobes), and isbroadened by a very modest fraction of 1.2 from 3.2 nm to 4.0 nmultimately delivering a relatively coarse axial resolution of 165 μm.

In light of the limitations of the current state of the art, there is aneed for low cost sources and detector arrangements for use ininterferometric imaging systems.

SUMMARY

The present application describes a source, with possible applicability,in one instance, to hybrid SS/SD-OCT, which achieves much greater powerthan today's SLDs and can be constructed from currently availableconsumer electronics at much lower cost. The source, among otherapplications, is particularly well suited to the low duty cycleillumination appropriate for field illumination OCT implemented withconsumer grade 2D array sensors with moderate to low frame rates. In oneembodiment, a single transverse mode semiconductor diode laser isoperated under non-traditional conditions for use in a low-coherenceinterferometric system. The optical wavelength output of the diode laseris swept over a range greater than 20 nm by applying pulses of drivecurrent, provided by a driver circuit, to the gain medium of the diodelaser. The shape of the pulse is optimized to drive the gain medium tovary the output wavelength of the laser over a spectral bandwidth thatis at least two times larger than the spectral output bandwidth underequilibrium conditions. In some instances, the spectral output bandwidthis at least five times larger than the spectral output bandwidth atequilibrium. The source may be realized using a low cost diode lasersuch as one that is commonly used for CD-R optical disk writing, whichis optimized for single transverse mode operation and pulsedcurrent >500 mA for pulses between 100 ns and 10 ms and outputwavelength at approximately 780 nm. Other semiconductor diode lasers arealso equally applicable for this application and may have benefits whichoutweigh an increased price. For instance, one semiconductor diode lasermay have a longer lasing cavity resulting in a tighter spectral combspacing and thus a smoother spectrum. The longer lasing cavity has thedrawback of requiring a higher pulse energy or accepting a narroweroverall bandwidth.

The wavelength swept source can be combined with a multiple pointimaging spectrometer for a hybrid SS/SD-OCT field sensing system forgenerating depth information of a sample. Optics illuminate a region ofa sample with the beam of radiation provided by the diode laser.Interference signal of the beam returning from the sample (possibly witha reference beam) is detected by the detector, which in one embodimentcould be a multiple point imaging spectrometer, and the observedinterference over the wide spectral range is converted into depthinformation of the sample. This arrangement compensates for somenon-ideal behavior of the source, and provides a data sampling solutionthat is well suited to the duty cycle requirements of the source.Minimum phase washout over a long integration time can be achieved bydriving a single current ramp over the exposure time of the spectrallyresolved detector. Typically the readout rate of the camera is the speedlimiting factor in SD-OCT. Alternatively, phase washout may be enhancedto act more like a traditional SD-OCT exposure by repeated sweeps acrossthe spectrum within the exposure window of the camera. This may beadvantageous for example to attenuate signal inside or below bloodvessels, which may increase contrast. This may also be useful tosuppress signal below the retinal pigment epithelium (RPE) of the eye.In one embodiment, a rolling shutter is described in combination withthe hybrid SS/SD-OCT system, where the exposure window is timed tocorrespond to the potentially exposed region of the sensor during thesweep.

The source and spectrometer design of the present application areparticularly well suited for line-field hybrid SS/SD-OCT, sparselysampled array OCT implemented on a 2D array, and line fieldself-interference interferometry using a spectrometer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a basic schematic of a generalized optical coherencetomography (OCT) system.

FIGS. 2(a) and 2(b) show operating characteristics of an exemplaryprior-art diode laser. In particular, FIG. 2(a) shows a plot of theoutput power vs. current of the diode laser. FIG. 2(b) shows a plot ofthe wavelength vs. temperature of the diode laser.

FIG. 3 shows an exemplary pulse ramp increasing from 0 A to 1 A ofcurrent over a period of 10 ms.

FIG. 4(a) shows the output of a diode laser operating under a drivecurrent pulse that is suitable for interferometric imaging using a 27kHz linear array spectrometer. FIG. 4(b) shows a time integratedspectrum based on the output shown in FIG. 4(a).

FIG. 5(a) shows an example distribution of heat flux without an AlNsubmount for a GaN laser diode in a TO 56 package. FIG. 5(b) shows thedistribution of heat flux with the AlN submount.

FIG. 6(a) illustrates a drive current vs. time chart when multiplepulses of drive current are delivered in a sequence. FIG. 6(b)illustrates the resulting time dependent junction temperature.

FIG. 7(a) is a plot of thermal impedance vs. time for a high power diodelaser. FIG. 7(b) shows the vertical temperature profiles of a singleemitter during various heating times.

FIG. 8 illustrates an exemplary line field hybrid SS/SD-OCT systemembodying a semiconductor diode laser light source discussed in thepresent application.

FIG. 9 shows plots of the sweep wavelength vs. the spectrum integrationtime for two example scenarios.

FIG. 10(a) illustrates a timing schematic of consumer grade CMOS 2Dsensor arrays operating in a rolling shutter mode. FIG. 10(b)illustrates a timing schematic of this operation in a ‘half-globalshutter’ mode. FIG. 10(c) illustrates a timing schematic of thisoperation in a ‘true global shutter’ mode.

FIG. 11(a) is a top-down view of a high-efficiency interferometerembodying a semiconductor diode laser light source discussed in thepresent application. FIG. 11(b) shows a side view of the high-efficiencyinterferometer of FIG. 11(a)

FIG. 12 illustrates an exemplary footprint of light when incident on adiffraction grating element of the interferometer in FIG. 11(b).

FIG. 13(a) is a side view of an example compact spectrometer design.FIG. 13(b) shows a side view of an alternative compact spectrometerdesign.

DETAILED DESCRIPTION

All patent and non-patent references cited within this specification areherein incorporated by reference in their entirety to the same extent asif the disclosure of each individual patent and non-patient referencewas specifically and individually indicated to be incorporated byreference in its entirely.

Definitions

The following definitions may be useful in understanding the detaileddescription:

Interferometric system: A system in which electromagnetic waves aresuperimposed in order to extract information about the waves. Typicallya single beam of at least partially coherent light is split and directedinto different paths. These paths are commonly called sample path andreference path, containing sample light and reference light. Thedifference in optical path length between the two paths creates a phasedifference between them, which results in constructive or destructiveinterference. The interference pattern can be further analyzed andprocessed to extract additional information. There are special cases ofinterferometric systems, e.g. common path interferometers, in which thesample light and reference light travel along a shared path.

Optical Coherence Tomography (OCT) System: An interferometric imagingsystem that determines the scattering profile of a sample along the OCTbeam by detecting the interference of light reflected from a sample anda reference beam creating a depth resolved (e.g., 2D/three-dimensional(3D)) representation of the sample. Each scattering profile in the depthdirection (z) is reconstructed individually into an axial scan, orA-scan. Cross-sectional images (B-scans), and by extension 3D volumes,are built up from many A-scans, with the OCT beam moved to a set oftransverse (x and y) locations on the sample. The axial resolution of anOCT system is inversely proportional to the spectral bandwidth of theemployed light source. The lateral resolution is defined by thenumerical aperture of the illumination and detection optics anddecreases when moving away from the focal plane. OCT systems exist intime domain and frequency domain implementations, with the time domainimplementation based on low coherence interferometry (LCI) and thefrequency domain implementation based on diffraction tomography. OCTsystems can be point-scanning, multi-beam or field systems.

Self-Interference Interferometry (SII) System: An interferometric depthranging system that reports the distribution of scatters in the objectin terms of the product of their reflectivities, and the distancebetween them. For instance, if the object contains a dominant scattereron one side of the scattering object, the output is functionally similarto OCT. Each scattering profile in the depth direction (z) isreconstructed individually into an axial scan, or A-scan.Cross-sectional images (B-scans), and by extension 3D volumes, are builtup from many A-scans, with the SII beam moved to a set of transverse (xand y) locations on the sample. The axial resolution of an SII system isinversely proportional to the spectral bandwidth of the employed lightsource. The lateral resolution is defined by the numerical aperture ofthe illumination and detection optics and decreases when moving awayfrom the focal plane. SII systems exist in frequency domainimplementations. SII systems can be point-scanning, multi-beam or fieldsystems.

Optical Coherence Domain Reflectometry (OCDR) System: A term referringto the coherent detection of the location and strength of a scattereralong a beam path, especially for the measuring of fiber lengths. Whenthe technique was extended to include 2D biological imaging, by scanningthe beam across a target and assembling the linear profiles into animage, it was commonly referred to as OCT; however, some academic groupsretain the original nomenclature for both the non-imaging technique andits derivative imaging technique.

Field illumination system: An interferometric imaging system wherein thesample is illuminated with a contiguous field of light which is thendetected with a spatially-resolved detector. This is in contrast toimaging systems which use a focused spot or multiple spatially-separatedfocused spots with a single detector for each spot. Examples of fieldillumination systems include line-field, partial-field and full-fieldsystems.

Point-scanning system: A confocal scanning system that transverselyscans the sample with a small spot and detects the backscattered lightfrom the spot at a single point. The single point of detection may bespectrally dispersed or split into two channels for balanced detection.Many points have to be acquired in order to capture a 2D image or 3Dvolume. Cirrus™ HD-OCT (Carl Zeiss Meditec, Inc. Dublin, Calif.) as wellas all other commercial ophthalmic OCT devices, are currentlypoint-scanning systems.

Parallel system: An interferometric imaging system that acquiresmultiple A-scans simultaneously at different positions across a field.The multiple A-scans might be directly adjacent in the case of fieldillumination, or may sparsely sample a field with distinctly separatedpoints which would need to be serially scanned in order to capture anapproximately continuous volume.

Line field system: A field illumination system that illuminates thesample with a line and detects backscattered light with a spatiallyresolved detector. Such systems typically allow capturing a B-scanwithout transverse scanning. In order to acquire an en face image orvolume of the sample, the line has to be scanned across the sample inone transverse direction.

Partial field system: A field illumination system that illuminates anarea of the sample which is smaller than the desired field of view anddetects the backscattered light with a spatially resolved detector. Inorder to acquire an enface image or volume of the entire desired fieldof view one requires transverse scanning in two dimensions. A partialfield illumination could be, for example, a spot created by a low NAbeam, a line, or any two-dimensional area including but not limited to abroad-line, an elliptical, square or rectangular illumination.

Full field system: A field illumination system that illuminates theentire desired field of view (FOV) of the sample at once and detects thebackscattered light with a spatially resolved detector. In order toacquire an enface image or volume, no transverse scanning is required.

Sparsely sampled array system: A highly parallel system in which manypoints over a wide field are sampled simultaneously. Distinct samplingpoints are sparsely distributed across the field (in contrast tocontiguous field illumination).

Photosensitive element: An element that converts electromagneticradiation (i.e. photons) into an electrical signal. It could be aphotodiode, phototransistor, photoresistor, avalanche photodiode,nano-injection detector, or any other element that can translateelectromagnetic radiation into an electrical signal. The photosensitiveelement could contain, on the same substrate or in close proximity,additional circuitry, including but not limited to transistors,resistors, capacitors, amplifiers, analog to digital converters, etc.When a photosensitive element is part of a detector, it is also commonlyreferred to as pixel, sensel or photosite. A detector or camera can havean array of photosensitive elements.

Detector: We distinguish between zero-dimensional (0D), one-dimensional(1D), and two-dimensional (2D) detectors. A 0D detector would typicallyuse a single photosensitive element to transform photon energy into anelectrical signal. Spatially resolved detectors, in contrast to 0Ddetectors, are capable of inherently generating two or more spatialsampling points. 1D and 2D detectors are spatially resolved detectors. A1D detector would typically use a linear array of photosensitiveelements to transform photon energy into electrical signals. A 2Ddetector would typically use a 2D array of photosensitive elements totransform photon energy into electrical signals. The photosensitiveelements in the 2D detector may be arranged in a rectangular grid,square grid, hexagonal grid, circular grid, or any other arbitraryspatially resolved arrangement. In these arrangements the photosensitiveelements may be evenly spaced or may have arbitrary distances in betweenindividual photosensitive elements. The 2D detector could also be a setof 0D or 1D detectors optically coupled to a 2D set of detectionlocations. Likewise a 1D detector could also be a set of 0D detectors ora 1D detector optically coupled to a 2D grid of detection locations.These detection locations could be arranged similarly to the 2D detectorarrangements described above. A detector can consist of severalphotosensitive elements on a common substrate or consist of severalseparate photosensitive elements. Detectors may further containamplifiers, filters, analog to digital converters (ADCs), processingunits or other analog or digital electronic elements on the samesubstrate as the photosensitive elements, as part of a read outintegrated circuit (ROIC), or on a separate board (e.g. a printedcircuit board (PCB)) in proximity to the photosensitive elements. Adetector which includes such electronics in proximity to thephotosensitive elements is in some instances called “camera.”

Spectrometer: A device for measuring light in a spectrally resolvedmanner Typically light is separated in angle according to wavelength bya diffractive or dispersive element and focused onto a spatiallyresolved detector, such that position on the detector encodes opticalwavelength. Other types of spectrometers which split the light by othermeans such as array waveguides, or even computationally such as withFourier Transform Spectrometers, exist. Spectrometers may measure asingle beam path or multiple beam paths. A spectrometer thatsimultaneously resolves multiple beam paths is frequently termed animaging spectrometer.

Light beam: Should be interpreted as any carefully directed light path.

Coordinate system: Throughout this application, the X-Y plane is theenface or transverse plane and Z is the dimension of the beam direction.

Drive Current: Electrons that flow across the active junction of asemiconductor laser device to stimulate the release of photons at theenergy associated with the bandgap of the active junction.

Current Pulse: A short-term increase in the drive current above abaseline level.

Pulse train: A sequence of current pulses.

Burst: A pulse train consisting of a small number of pulses in rapidsuccession

Speckle diameter: A region over which an observation of interference hashighly correlated phase. This measure is highly correlated with theresolution and point spread function of a interferometric imaging system(see for example, Schmitt, J. M., Xiang, S. H., and Yung, K. M. (1999).Speckle in Optical Coherence Tomography. J. Biomed. Opt 4, 95-105).

General OCT System

A diagram of a generalized ophthalmic OCT system is shown in FIG. 1. Abeam of light from source 101 is routed, typically by optical fiber 105,to illuminate the sample 110, a typical sample being tissues in thehuman eye. The source 101 is typically either a broadband light sourcewith short temporal coherence length in the case of SD-OCT or awavelength tunable laser source in the case of SS-OCT. The light isscanned, typically with a scanner 107 between the output of the fiberand the sample, so that the beam of light (dashed line 108) is scannedlaterally (in x and y) over the region of the sample to be imaged. Lightscattered from the sample is collected, typically into the same fiber105 used to route the light for sample illumination. Reference lightderived from the same source 101 travels a separate path, in this caseinvolving fiber 103 and retro-reflector 104 with an adjustable opticaldelay. Those skilled in the art recognize that a transmissive referencepath can also be used and that the adjustable delay could be placed inthe sample or reference arm of the interferometer. Collected samplelight is combined with reference light, typically in a fiber coupler102, to form light interference that is observed using a detector 120.Although a single fiber port is shown going to the detector, thoseskilled in the art recognize that various designs of interferometers canbe used for balanced or unbalanced detection of the interference signal.The output from the detector 120 is supplied to a processor 121 thatconverts the observed interference into depth information of the sample.The results of the processing can be stored in the processor 121 orother storage medium or displayed on display 122. The processing andstoring functions may be localized within the OCT instrument orfunctions may be performed on one or more external processing units towhich the collected data is transferred. This unit could be dedicated todata processing or perform other tasks which are quite general and notdedicated to the OCT device. The processor 121 may contain for example afield-programmable gate array (FPGA), a digital signal processor (DSP),an application specific integrated circuit (ASIC), a graphics processingunit (GPU), a system on chip (SoC) or a combination thereof, thatperforms some, or the entire data processing steps, prior to passing onto the host processor or in a parallelized fashion.

The sample and reference arms in the interferometer could consist ofbulk-optics, fiber-optics or hybrid bulk-optic systems and could havedifferent architectures such as Michelson, Mach-Zehnder or common-pathbased designs as would be known by those skilled in the art. Light beamas used herein should be interpreted as any carefully directed lightpath. In time-domain systems, one arm of the interferometer typicallycontains a tunable optical delay to generate a time variable ramp in theinterference phase across the spectrum. Balanced detection systems aretypically used in TD-OCT and SS-OCT systems, while spectrometers areused at the detection port for SD-OCT systems.

In Fourier Domain optical coherence tomography (FD-OCT), eachmeasurement is the real-valued spectral interferogram (S_(j)(k)). Thereal-valued spectral data typically goes through several post-processingsteps including background subtraction, dispersion correction, etc. TheFourier transform of the processed interferogram, results in a complexvalued OCT signal output A_(j)(z)=|A_(j)|e^(iφ). The absolute value ofthis complex OCT signal, |A_(j)|, reveals the profile of scatteringintensities at different path lengths, and therefore scattering as afunction of depth (z-direction) in the sample. Similarly, the phase,φ_(j) can also be extracted from the complex valued OCT signal. Theprofile of scattering as a function of depth is called an axial scan(A-scan). A set of A-scans measured at neighboring locations in thesample produces a cross-sectional image (tomogram or B-scan) of thesample. A collection of B-scans collected at different transverselocations on the sample makes up a data volume or cube. For a particularvolume of data, the term fast axis refers to the scan direction along asingle B-scan whereas slow axis refers to the axis along which multipleB-scans are collected. A variety of ways to create B-scans are known tothose skilled in the art, including but not limited to scanning thesample beam along the horizontal or x-direction, along the vertical ory-direction, along the diagonal of x and y, or in a circular or spiralpattern.

Pulse Tuning the Source

In one aspect of the present application, a common semiconductor diodelaser is supplied with a drive current such that its spectral output isoptimized for interferometry applications such as OCT, OCDR, or SII. Anexemplary diode laser for this purpose is LNCT28PF01WW produced byPanasonic, described as an edge emitting, Fabry-Perot, 780 nm band diodelaser with multi quantum well structure. The laser is selected forhaving a wavelength appropriate to a given task, for example ophthalmicimaging of the eye (in this case, a wavelength with good waterpenetration and low visual excitation potential are desirable).Preferably, the laser should have a significant and approximatelycontinuous dependence of wavelength on operating temperature. Theoptical path length of the laser cavity (i.e., its physical lengthmultiplied by its refractive index) is preferably long, such that thelongitudinal modes are closely spaced. The end facets of the laser arepreferably optimized for high peak optical power. Preferably, the laseroutputs a single spatial mode so any portion of the sampled beam can beexpected to interfere with high contrast with another part of a sampledbeam. Finally, the laser is preferably used in a high volume applicationand therefore is likely to be available at very low cost. The compactdisk read-write application offers the combination of compatiblewavelength range, high-power, single transverse mode pulses, as well asa very large and price sensitive market.

FIGS. 2(a) and 2(b) show the published operating characteristics of thediode laser LNCT28PF01WW by Panasonic. FIG. 2(a) shows a plot of theoutput power vs. current while FIG. 2(b) shows a plot of the wavelengthvs. temperature of the diode laser. The maximum recommended continuouswave (CW) current is about 300 mA. Driving the slightly cooled diode atminimum CW current results in an output wavelength of less than 780 nm,which gives a good indicator of the minimum wavelength practical withthis diode. Driving the diode near its maximum CW current at its maximumoperating temperature should drive the junction near to its operatingtemperature limit resulting in a wavelength greater than 800 nm.

The diode laser light source typically has a spectral output bandwidth<1 nm when driven under equilibrium conditions (constant current andtemperature). The diode laser is operably attached to a driver circuitcapable of applying pulses of drive current to the diode laser. For theembodiments described herein, the drive current pulse is designed toproduce an output spectrum of suitable shape for interferometricapplications when integrated over time. To achieve such a usable shapeoutput spectrum, certain design considerations should ideally beachieved, some of which are discussed as follows:

-   -   1. The pulse of drive current should cause the laser active        junction to achieve a significant widening of the laser        bandwidth, with the increase in bandwidth being ideally at least        as large as the spectral output bandwidth under equilibrium        operating conditions as described above (thus doubling the        spectral output bandwidth). In some instances, the spectral        output bandwidth is at least five times larger than the spectral        output bandwidth at equilibrium. Most likely this spectral shift        is due to temperature changes. An increase in temperature causes        a decrease in the material bandgap resulting in a shift of the        peak gain of +0.3 nm/C for GaAs (see for example, Bartl, J.,        Fíra, R., and Jacko, V. (2002). Tuning of the laser diode.        Measurement Science Review volume 2 section 3). Ideally the        temperature sweep during a drive current pulse covers a range        from a lower baseline temperature to a point near an upper        practical operating limit of the laser. The lower baseline        temperature may be actively cooled below the ambient temperature        to increase the available temperature range. The upper operating        limit of the laser may be defined either as a temperature where        the laser efficiency is highly reduced, or where running the        laser hotter would reduce its lifetime beyond usability for the        application.    -   2. The pulse of drive current should deliver a significant        amount of optical energy while operating at each wavelength so        as to produce a usable shaped time integrated spectrum. Joule        heating most likely causes the temperature of the active        junction to rapidly rise to an approximate equilibrium. To first        order, this temperature rise can be described by        ΔT(t)=T_(o)[1−exp(−t/τ)], where τ is a thermal time constant of        the diode junction typically on the order of a few hundreds of        nanoseconds. The thermal time constant is the time required for        junction to reach 1−1/e˜63.2% of the final change in temperature        upon a step increase in current. The output spectrum of the        laser may then provide a good estimate of the junction        temperature. Frequently an approximately Gaussian shaped        spectrum is considered desirable.    -   3. The pulse of drive current should cause cavity length shifts        which change the position of the laser lines enough to blur the        line structure in the time integrated spectrum. The cavity        length shift is a result of index of refraction changes due to a        combination of injection current effects and temperature        changes.    -   4. The pulse duration should be sufficiently short as to avoid        any artifacts such as fringe washout as described in the        Background section. For a preferred embodiment of a hybrid        SS/SD-OCT system, the preferred pulse duration would likely be        shorter than 1 ms while for an SD-OCT system, the preferred        pulse duration would likely be shorter than 100 μs.    -   5. The characteristics of the drive current, such as the        temporal profile, the peak current, and the repetition rate,        should be designed in a way that pulsing the diode laser does        not damage it or significantly reduce its lifetime for any        practical OCT application. Because of the low cost of the diode        laser, several hundred hours of operation are likely possible        since users can replace with a new one if it is damaged.

The temporal profile of the current pulse may comprise a polynomial, anexponential function, or any other time-varying shape that could meetthe above goals. Likely examples include a rectangle or a linear ramp.For instance, a short pulse of approximately constant current, ofduration on the order of the thermal time constant of the junction orless, is one such way to achieve a spectrum that is approximately flat(because the temperature vs time curve is approximately linear in thisshort time period). The equilibrium temperature for the approximatelyconstant current is significantly larger than the final temperature atthe end of the linear temperature range. If such a pulse achieves alarge change in temperature, the equilibrium temperature may be veryhigh, potentially beyond the temperature required to destroy the diode,such as by melting the solder layers. Therefore, the diode is operatedin a regime where the peak junction temperature is critically dependenton the duration of the pulse, which should be less than a few hundredsof nanoseconds, depending on the structure of the diode.

The rectangular pulse described above has relatively small energybecause the pulse duration is very short. In order to increase theamount of energy, a pulse train consisting of many short pulses can beconstructed. Such short, critically timed pulses may be created withsmall modification to circuitry used to drive compact disk write cycles,which typically deliver high current pulses for durations on the orderof 500 ns or shorter.

An alternative pulsing regime that also produces a usable shaped timeintegrated spectrum, drives the current with a slow pulse so that thejunction temperature stays very close to the equilibrium temperatureassociated with the drive current at all times. The pulse may preferablybe shaped to produce a desired time integrated spectrum. Because, 1) theoptical output power is approximately proportional to current, 2) theheating is approximately proportional to the current squared, and 3) thetemperature change is approximately proportional to the heating, adesirable shape for the current ramp tends to have an increasing slopeat high currents. The shape of the current ramp can be furthercontrolled to shape the time integrated spectral energy envelope to adesired form such as flat top, Gaussian or Hamming, for the purposes ofachieving a desired axial point spread function with minimum noise andpost process modification.

FIG. 3 shows an exemplary pulse ramp increasing from 0 A to 1 A ofcurrent over a period of 10 ms. This current ramp is sufficient to causethe LNCT28PF01WW diode laser (described earlier with respect to FIGS.2(a) and 2(b)) to sweep over an optical bandwidth of more than 20 nm.Such a low electrical bandwidth pulse is easy to generate with a lowcost current driver modulated by input voltage. The input current rampmay be created by analog or digital waveform synthesis such as simpleresistor-capacitor circuits in combination with TTL chips such as asimple 555-timer.

FIG. 4(a) shows the output of a diode laser under a drive current pulsesuitable for interferometric imaging using a 27 kHz linear arrayspectrometer to resolve the swept spectrum vs time. Time increases fromtop to bottom in the figure. Individual longitudinal modes form shortbright line segments across the time vs wavelength space. A nearlyvertical line (401) indicates that a resonant mode has a nearly constantwavelength. A line with significant horizontal tilt (402) indicates thatthe resonance is shifting because of index of refraction changes whichcause a slight shift in the apparent length of the laser cavity. Up toseveral longitudinal modes of the laser may be simultaneouslyilluminated at a given drive current (405). The illuminated modes sweeptoward longer wavelengths (right) as the laser junction heats up underincreased drive current (bottom). The total swept bandwidth (406) ismuch greater than the bandwidth at any fixed current. The exposure timeassociated with any particular wavelength (407) is much shorter than thetotal exposure time (408).

FIG. 4(b) shows the time integrated spectrum from the experimental datashown in FIG. 4(a). The horizontal axis is displayed in pixel numberwhich is proportional to wavelength. Pixel 200 is approximately 785 nm.Pixel 1300 is approximately 810 nm. At shorter wavelengths associatedwith very low current, line spectra associated with unblurred modes arehighly visible. At longer wavelength associated with greater current,distinct modes have blurred to fill in the complete spectrum.

In typical interferometric imaging systems like OCT, OCDR and SII, it isdesirable to acquire repeated measurements. Therefore, we consider notonly the characteristics of a single pulse, but also a sequence ofpulses used to operate the device. During the pulse, heat is generated,which must eventually be dissipated to the environment, but is firstdistributed to the elements of the diode package, typically including:the rest of the semiconductor chip, electrical solders and connections,and submounts. FIG. 5(a) shows the distribution of heat flux without analuminum nitride (AlN) submount for a GaN laser diode in a TO 56 package(see for example, Feng, M.-X., Zhang, S.-M., Jiang, D.-S., Liu, J.-P.,Wang, H., Zeng, C., Li, Z.-C., Wang, H.-B., Wang, F., and Yang, H.(2012). Thermal analysis of GaN laser diodes in a package structure.Chinese Physics B 21, 084209). FIG. 5(b) shows the distribution of heatflux with the AlN submount. The temperature of the active junction(e.g., at the bottom surface of the chip) will generally not reducebelow the temperature of the bulk material which is limited by theaverage current. To reach high temperatures, it is desirable to have ahigh peak current; on the other hand, to drive the junction over a widerange of temperatures, the junction should have a low baselinetemperature to start at, or return to, therefore, the average currentwould ideally be kept low. This implies delivering pulses with a loweffective duty cycle. The baseline temperature to which the junctionreturns to will be limited by the average current during a timecorresponding to a characteristic time of the device. Operation of manypulses with high duty cycle will result in an upward creepingtemperature, shift towards longer wavelength, and decreasing efficiencyof the diode laser.

FIG. 6(a) illustrates a drive current vs. time chart when multiplepulses of drive current (601) are delivered in a sequence. The resultingtime dependent junction temperature is illustrated on the lower chart inFIG. 6(b). On multiple pulses in sequence, the minimum temperature (602)drifts toward a temperature associated with the mean drive current inthe pulse train. A long recovery period (603) between bursts allows thetemperature to return to a lower baseline (604). Increasing time betweenbursts or pulses can decrease the baseline temperature and allow a totalgreater bandwidth sweep.

Understanding the thermal characteristics of the diode laser will helpdesign optimal drive current pulse trains to achieve optimal bandwidthand duty cycle. Each element of the package can be characterized by itsthermal resistivity and thickness. These characteristics describe thetime required for heat to move from a warmer area to a cooler area. Ingeneral, one can think of the temperature of parts of the package whichare close to the active area as responding very rapidly to theinstantaneous current, whereas parts which are farther away respond moreslowly and appear to follow the average current. Frequently the packagedesign is such that the largest resistance to heat flow is relativelydistant from the active junction. If the pulses are long relative to thethermal equilibrium time of the active junction, the temperature of thediode is well controlled, without tight requirements on the timingprecision of the pulse. If the pulses are short relative to the thermalequilibrium time of larger parts of the diode package, the small partscan cool rapidly because the heat has not significantly raised thetemperature of the bulk heatsink.

FIG. 7(a) is a plot of thermal impedance vs. time for a high power diodelaser without voids in the solder layer (see for example, Suhir, E.,Wang, J., Yuan, Z., Chen, X., and Liu, X. (2009). Modeling of thermalphenomena in a high power diode laser package. In Electronic PackagingTechnology & High Density Packaging, 2009. ICEPT-HDP'09. InternationalConference on, (IEEE), pp. 438-442). The bottom panel 704 in FIG. 7(a)is a zoomed in view of the data in the top panel 702. FIG. 7(b) showsthe vertical temperature profiles of a single emitter during variousheating times. The figures show that heat flows easily from the junctionfor short pulses, but becomes more difficult as the package becomeswarmer from a history of usage. That is, the laser will cool rapidly andnearly completely from a single short pulse, but recovery from a longpulse train will take more time.

The spectral bandwidth sweep achievable with the present technique istypically less than 30 nm, which is somewhat less than the spectralbandwidth achieved by modern SLDs. The range may be extended by coolingof the package below room temperature or by combining multiple diodeswith neighboring spectral ranges.

Use of Pulse Tuned Source in a Double-Pass Interferometric System

The swept diode laser source described in the previous section isoptimally integrated into a depth ranging system which makes good use ofits potentially high output power, swept wavelength characteristics, andis compatible with its low duty cycle. A first preferred embodiment isconfigured as a line field hybrid SS/SD-OCT system as illustrated inFIG. 8. The hybrid configuration accommodates the non-idealinstantaneous linewidth of the source, which has been observed to be farfrom ideally narrow at portions of the sweep, while the swept nature ofthe source allows for a long exposure time for maximum signal. Thesemiconductor diode laser (801) light is spread by asymmetric optics(803) such as cylinder or Powell lenses to create a line ofillumination. The light beam (805) is split by a beamsplitter (807) andone path becomes a reference path which is reflected by use of opticalcomponents (809). Depending on the application, the optics in 809 mayalso contain asymmetric optics such as a cylindrical or Powell lens. Theother path from the beamsplitter is directed to a sample (813) throughoptics and an optional scanning mechanical component (811). As aline-field system provides a single B-scan for each exposure of thecamera, it may not be required or even desired to provide volumetricscanning through a scanner. Such systems would further decrease theoverall cost and physical footprint. Light from the illuminated sampleis imaged onto the optional entrance slit (815) of a spectrometer (817).Light from a reference arm simultaneously illuminates the optionalentrance slit of the spectrometer 817, and interferes with the lightfrom the sample 813. The spectrometer 817 disperses the lighttransmitted through the slit 815 by wavelength onto a 2D area detector(camera) (819). The modest frame rate of the low cost 2D array providesa good effective A-scan rate, while being compatible with the low dutycycle of the source. A processor (such as the processor 121 (see FIG.1)) converts the recorded spectrum into B-scan data, which may befurther be processed to produce depth maps etc. and displayed on adisplay (e.g., the display 122 in FIG. 1)

Because of the potential for high power, the desirability of low dutycycle, the instantaneous spectral characteristics, and the cost of thissource; it is particularly well suited to highly parallel spectraldomain implementations with a 2D array detector. Line-field approachesenabled by this source are particularly well suited to performancerequirements of minimum cost applications including slit-lamp based OCTand self-interference OCT. Sparsely sampled array OCT as described byAnderson et al. (see for example, Anderson, T., Segref, A., Frisken, G.,& Frisken, S. (2015, March). 3D spectral imaging system for anteriorchamber metrology. In SPIE BiOS (pp. 93120N-93120N), InternationalSociety for Optics and Photonics) is also well suited to this source.Although not all of the synergies noted for the above configurations maybe present, the source may also be used as a low cost source forpoint-scanning SD-OCT (see for example, Yun, S. H., Tearney, G., deBoer, J., & Bourna, B. (2004). Pulsed-source and swept-sourcespectral-domain optical coherence tomography with reduced motionartifacts. Optics Express, 12(23), 5614-5624), point scanning parallelSD-OCT (see for example, Anderson, T., Segref, A., Frisken, G., &Frisken, S. (2015, March). 3D spectral imaging system for anteriorchamber metrology. In SPIE BiOS (pp. 93120N-93120N). InternationalSociety for Optics and Photonics), line-field SS-OCT (see for example,Grajciar B., Pircher M., Fercher A., and Leitgeb R. (2005). ParallelFourier domain optical coherence tomography for in vivo measurement ofthe human eye. Optics Express 13, 1131-1137; Nakamura, Y., Makita, S.,Yarnanari, M., Itoh, M., Yatagai, T., & Yasuno, Y. (2007). High-speedthree-dimensional human retinal imaging by line-field spectral domainoptical coherence tomography. Optics express, 15(12), 7103-7116; andFechtig, D. J., Grajciar, B., Schmoll, T., Blatter, C., Werkmeister, R.M., Drexler, W., & Leitgeb, R. A. (2015). Line-field parallel sweptsource MHz OCT for structural and functional retinal imaging. Biomedicaloptics express, 6(3), 716-735), full field SS-OCT (see for example,Bonin, T., Franke, G., Hagen-Eggert, M., Koch, P., & Hüttmann, G.(2010). In vivo Fourier-domain full-field OCT of the human retina with1.5 million A-lines/s. Optics letters, 35(20), 3432-3434.), full fieldTD-OCT (see for example, Watanabe, Y., Hayasaka, Y., Sato, M., & Tanno,N. (2005). Full-field optical coherence tomography by achromatic phaseshifting with a rotating polarizer. Applied optics, 44(8), 1387-1392),low-coherence holography (see for example, Girshovitz, P., and Shaked,N. T. (2014). Doubling the field of view in off-axis low-coherenceinterferometric imaging. Light Sci Appl 3, e151), or any otherinterference technique where it is advantageous to use a low cost, highpower, source with significant bandwidth.

The source is pulsed to produce a single sweep of a durationcorresponding to approximately the maximum time where motion artifactsare rare or manageable. For the human eye this pulse length with a sweptsource is a little longer than 1 ms (see for example, Fechtig, D. J.,Grajciar, B., Schmoll, T., Blatter, C., Werkmeister, R. M., Drexler, W.,& Leitgeb, R. A. (2015). Line-field parallel swept source MHz OCT forstructural and functional retinal imaging. Biomedical optics express,6(3), 716-735). For a hybrid SS/SD-OCT system, we would expect thepreferred pulse duration to thus be less than 1 ms. For most samples,the ratio of this pulse length to the frame period of the camera(limited to about 100 Hz in most low cost 2D arrays used in consumercameras) provides a duty cycle of a few percent which is highlycompatible with maintaining a wide bandwidth thermal sweep. Thereference arm may be incident on axis or off axis (see for example, USPatent Publication No. 2014/0028974) depending on the desiredspectrometer and sensor resolution tradeoff. Spectral resolutionprovided by the grating spectrometer can be superior to the spectralresolution which would be attained by using the source as a timeresolved swept source, because the source frequently supports multiplelongitudinal modes with a significant instantaneous bandwidth.Calibration of the spectrometer is also straightforward, whereascalibration of the sweeping source may be quite complex as the tuningcurve is not strictly monotonic.

Duration of Laser Pulse and Camera Exposure

The pulse may also be optimized to produce an optimal interferencemeasurement. To maximize signal intensity, it is desirable to maximizethe number of photons returned from the sample with a constantinterference phase. In measurements of living systems, motion of thesample typically limits the amount of time in which an observation canhave a constant interference phase. Axial motion of the sample by adistance of one quarter of a wavelength of light relative to a referencecauses a phase difference of 180 degrees, completely reversing thenature of interference, and potentially eliminating signal during themeasurement period. In addition, lateral motion by a significantfraction of the speckle diameter causes a decorrelation of the phasebetween the beginning of the measurement period and the end of themeasurement period with similar loss of signal results. Such cancelationof signal during a measurement period is called phase washout. Inmeasurement systems with a low numerical aperture, such as the humaneye, the speckle diameter is much larger than the wavelength of light.As a result, motion in the axial direction is typically the morelimiting case. It is typically desirable to take OCT measurements fastenough to avoid washout, although other cases do exist (e.g. opticallock-in). Axial phase washout is determined by the amount of motionexperienced during the exposure time associated with any singlewavelength. In the case where we wish to minimize phase washout, it isbest to drive the laser to achieve a single sweep of wavelength acrossthe measurement integration time period. In this way, each wavelengthsample is measured for a small fraction of the total sweep time. In thecase where we want to maximize the phase washout, we may drive the laserthrough many sweeps during a single integration period. In this way,each wavelength is sampled at multiple time points over a relativelylong measurement time. Ideally a large number of sweeps is included suchthat the signals are likely to cancel completely if there is significantmotion. Similarly, if the source is used in time domain applications,the pulse modulation rate should be fast compared to the phasemodulation between the sample and reference arms.

FIG. 9. shows plots (902) and (904) of the sweep wavelength vs. thespectrum integration time for two different scenarios. In the normalcase such as the one depicted in plot (902), phase washout is minimizedby actuating a single wavelength sweep over an integration period. Inrare cases such as the one depicted in plot (904), where it isadvantageous to introduce phase washout, the wavelength may be sweptmultiple times during a single integration period.

Many consumer grade CMOS 2D sensor arrays are operated in what is knownas rolling shutter mode. A timing schematic of this operation isdepicted in FIG. 10(a). In rolling shutter mode, the entirety of eachframe in the capture sequence is not exposed from exactly the same startand end time, but the integration start and end is rastered across thesurface of the detector. Rolling shutter mode is advantageous in thehybrid swept-source, spectral domain system described herein tosynchronize the exposure window of the camera with the portion of thearray that is being instantaneously illuminated by the swept source. Theorientation and timing of the rolling shutter can be optimized to bestmatch the illumination of the sensor as it traverses across the surfaceof the sensor according to its wavelength sweep and the dispersion ofthe spectrometer. Preferably, the read out raster direction is orientedsuch that elements of the same wavelength are read out as close tosimultaneously as possible. Also preferably, the read out rasterdirection is oriented such that the wavelengths that are produced firstin a sweep of the laser are read out first. That is, the direction ofthe rolling shutter is in the same direction as the sweeping of thelight across the detector. And preferably, the shutter is opened on aregion of the detector shortly before that region of the detector isilluminated by the wavelength sweep, and is closed shortly after thatregion of the detector is illuminated by the wavelength sweep, therebyminimizing the detector dark current by keeping the integration timeshort compared to the frame rate of the camera and the duration of thelaser sweep. An integration time that is windowed closely to thewavelength sweep of the laser also limits the effect of stray light inthe spectrometer. The width of the integration time should be wideenough to allow for the maximum instantaneous spectral width of thesource, as well as to accommodate tolerance for sweep ratenon-linearities, timing uncertainties, and projection distortions acrossthe detector. There may also be a pause between two sweeps to allow forthe diode laser to recover if necessary. Two diode lasers could also beinterleaved to allow one to rest on odd sweeps and the other to rest oneven sweeps.

During a rolling shutter exposure (FIG. 10(a)), the exposure of multiplerows are overlapped and thus it is not necessary for the sweep speed toexactly match the rolling shutter speed. In cases where the readout rateis limited by the sensor array, and the source sweep time is very shortcompared to the maximum readout period of the sensor, a small reductionin the shutter open time can be achieved by preferably orienting thereadout direction relative to the wavelength sweep direction asdescribed above. In this case, the laser sweep should be initiated witha delay after the open shutter signal begins to cross the detector, suchthat the sweep will finish just as it catches up with the open shuttersignal. The close shutter/readout signal may begin approximatelysimultaneously with the beginning of the wavelength sweep of the sourceand will continuously lag behind the sweeping light source due to itsslower rate across the sensor.

Another suitable configuration of the camera is what is called‘half-global shutter’ mode and a timing schematic of this operation isdepicted in FIG. 10(b). In this mode, all pixels on the camera are resetand begin exposure simultaneously. During this time, the sample isilluminated which, in our setup refers to one or more sweeps of thesemiconductor diode laser. Once the illumination is completed, each rowof pixels is read out one-by-one while the other rows continue to beexposed. This mode of operation exists because the minimalcharge-transfer and readout electronics on the camera only allows forone row to be captured and transferred at a time. In contrast to therolling shutter mode as depicted in FIG. 10(a), this mode allows forvery short sweep times with long pauses between pulses. One disadvantageis the increased noise characteristics when compared to a true globalshutter or a rolling shutter.

A timing schematic of a ‘true global shutter’ mode is depicted in FIG.10(c). In this mode, all pixels begin and end exposure at the same time.Extra electronics (when compared to rolling shutter or half-globalshutter) allow for the charge from each pixel to be transferred andstored temporarily while each row is read out one-by-one. This mode ofoperation is not commonly found in low-cost 2D sensors.

Image Processing Techniques

When using the semiconductor diode laser source discussed herein in aninterferometric system (for example, with respect to FIG. 8), it isoften desirable to perform background subtraction to remove anylow-frequency variations introduced by the reference beam. Because ofthe diode laser ideally being driven under loosely controlledconditions, or under repeated pulses in non-equilibrium conditions, thespectrum shape may vary significantly from cycle to cycle. Paralleldetection arrangements provide additional advantages in processing anunstable spectrum.

One possibility is for a detector channel to sample the source spectrumwithout interference in parallel with the usual interferometricdetection. In a line-field hybrid SS/SD-OCT system, for instance, thiscould be achieved by blocking the sample light from hitting part of the2D detector. Using this area and knowledge of the varying PSF across thedetector, the background spectrum could be accurately estimated acrossthe detector. Another possibility is to use multiple detection channelswith interference to estimate the source spectrum on a per sweep basis.If these measurements vary in some way (be it the phase delay, spatialsampling location, spatial sampling angle, etc.), the mean (or someother combination) of some or all of the A-scan channels could be usedas an estimate of the spectrum. A variety of other estimation approachesare possible which include, but are not limited to, median,maximum-likelihood estimation, and estimation filters (linear ornon-linear) such as the Kalman filter.

To improve the shape of the acquired spectrum, the interference spectrumassociated with each A-scan can be divided by the estimate of the sourcespectrum to create a normalized spectrum, and multiplied by a windowingfunction to optimize spectral shape, therefore optimizing the axialpoint spread function in the reconstructed image. In some regions of thespectrum where the peaks modulate to large degrees, spectralnormalization may be highly susceptible to noise. Thus, it may bebeneficial to use the windowing function to partially attenuate thesewavelengths which allows for the smoother portions of the spectrum todominate. The order of these multiplicative steps is generally somewhatmutable and may be combined with other steps including resampling,dispersion compensation, background subtraction, etc. and in some casesmay even be applied (as a convolution filter) on the other side of aFourier transform or filter bank operation.

One computationally expensive step in OCT image processing is resamplingthe acquired data to be linear-in-k space. Such a step is typicallyperformed using interpolation algorithms such as nearest neighbor,linear, quadratic, cubic, spline, sinc, etc. Especially for a low-costOCT system, minimizing the required computational power is desirable.The less computationally expensive interpolation methods typicallyprovide lower image quality. Somewhat unique to this low-cost OCT systemis the small to moderate measured optical bandwidth. As such, thenon-linearity in the bandwidth should be small and thus it may bepossible to utilize simpler interpolation techniques than forhigher-performance systems (perhaps as simple as a nearest-neighborinterpolation). Furthermore, provided with the small optical bandwidth,and the availability of detectors with a large number of pixels (such as1024 or more pixels), a large imaging depth is easily obtainable,minimizing the possibility of image aliasing during the interpolationstep. This can further simplify the interpolation technique.

Use of Pulse Tuned Source with a High-efficiency Interferometer

An alternative embodiment to the one in FIG. 8 is depicted in FIGS.11(a) and 11(b). In this embodiment shown from a top view in FIG. 11(a)and from the side view in FIG. 11(b), a high-efficiency interferometerdesign including beamsplitter (1100) is used to maximize both the powerto the sample and the collected light from the sample (1120). In atraditional setup e.g. FIG. 8, the collected light passes through thebeamsplitter (807) twice (once during illumination and once duringcollection). For a 50/50 beamsplitter, 75% of the light is thus lost(50% for each pass). In some instances, a different splitting ratio suchas 90/10 can be utilized to illuminate with 10% of the source andcollect 90% of the backscattered light from the sample (see for example,Shemonski, N. D., et al. “Computational high-resolution optical imagingof the living human retina.” Nature Photonics 9, 440-443, 2015), but anuneven splitting ratio where a small amount of light is directed to thesample typically makes sense when the total amount of light incident onthe sample needs to be limited.

For a line-field system, a much higher total power can be incident onthe sample, meaning that diverting less light to the sample is notideal. In the configuration shown in FIGS. 11(a) and 11(b), most of theinput light, e.g. 90% or even 99% can be directed to the sample (1120),then backscattered light from the sample bypasses the beamsplitter(1100) possibly through an aperture in a plane conjugate (or close toconjugate) to a Fourier plane relative to the sample. The beam directedto the sample 1120 is denoted by 1105 a and a single plane-wave of thebackscattered light collected from the sample 1120 is denoted by 1105 b.This plane in the optical system, and its imaging conjugates, arereferred to as ‘Fourier planes’ where the spatial distribution of lightcorresponds to the angular distribution of light in the sample plane atbest focus. In a simplified optical system where the best focused planein the sample lies at the front focal plane of an imaging lens, thisFourier plane lies at the back focal plane of the same lens. Thereference beam (1102) first transmits through the beam splitter thenreflects off a mirror (1103) and then is reflected off the beamsplitter(1100) on return. Since the collected sample light and the referencebeam are spatially separated in the Fourier plane, a linear phase rampwill be introduced between the sample and reference in a plane conjugateto the sample (e.g. 1104). Due to the phase ramp, the interferencefringes along the line of illumination will be modulated with a carrierfrequency and can thus be used to remove one or more of the following:conjugate mirror image, autocorrelation signal, and need for backgroundsubtraction (see for example, US Patent Publication No. 2014/0028974).

When designing such a system for ophthalmic imaging, it might bedesirable to place the pupil of the eye at or near to a plane conjugateto the beamsplitter (1100). In a design such as that depicted in FIG.11(a), this means that the illumination and collection light arespatially separated on the pupil. To minimize the required size of thepupil and to make alignment with the subject easier it is desirable tominimize the distance between the illumination and collection beams.This can be taken to the extreme where the illumination and collectionbeams are again overlapped as in a traditional on-axis OCT system. Tostill efficiently remove the mirror image, autocorrelation signal, andthe background signal, the reference arm can be offset using mirrors,lenses, or other optics causing the returning reference beam to notstrike exactly the same place on the beamsplitter where it was divided(see for example, Fechtig, D. J., Grajciar, B., Schmoll, T., Blatter,C., Werkmeister, R. M., Drexler, W., & Leitgeb, R. A. (2015). Line-fieldparallel swept source MHz OCT for structural and functional retinalimaging. Biomedical optics express, 6(3), 716-735). The optics in thereference arm can also cause the reference beam to fully bypass thebeamsplitter resulting in a setup similar to a Mach-Zehnderinterferometer. Utilizing the aforementioned approaches effectivelydecouple the off-axis detection angle from the beam separation in thepupil of the eye. This also applies to non-ophthalmic systems where anaperture exists near a plane conjugate to the Fourier plane of thesample.

In order to use a system as diagramed in FIGS. 11(a) and (b) to image ahuman retina, the sample optics would be modified to image the apertureplane (containing the high efficiency beamsplitter) to the pupil of thehuman eye, and allow the optics of the eye to focus the light onto theretina, and the length of the reference arm would be increased to matchthe full optical length to the retina. One way the sample arm optics canbe simply realized is to add an objective lens in the sample pathforming a 4-f optical system where the distance between the imaging lensand the objective lens forms a Badal optometer, which can compensate forthe refractive error of the patient by making small adjustments to thedistance between the two lenses. The footprint in the pupil of the eyeis an image of the aperture stop of the system containing the highefficiency beamsplitter, so the pupil of the eye can be thought of as anillumination pupil and an adjacent collection pupil.

A side-view of a portion of the system in FIG. 11(a), including thespectrometer, is shown in FIG. 11(b) where the Fourier plane relative tothe sample on the beamsplitter (1100) is relayed (via optics 1110 and1112) to the diffraction grating (1108). The grating can be of thereflective or transmissive type. An optional entrance slit (spatialfilter, 1106) can be placed between these components. The diffractedlight is then focused onto a 2D sensor 1114 (via optics 1116). Separatewavelengths will be spread along one dimension and spatial informationalong the other. This forms an imaging spectrometer.

The footprint of light incident on the grating is as indicated in FIG.12. The sample and reference beams are spatially separated by theaperture at the Fourier plane relative to the sample, and are thusseparated on the grating where the reference beam footprint at thegrating is denoted 1204 and the sample beam footprint at the grating isdenoted 1202 (the grating is conjugate, or near conjugate, to a Fourierplane relative to the sample). When propagated to the sample plane (fromthe grating, through a lens, onto the sensor), the separation willresult in a linear phase ramp between the reference and sample light.

The elliptical-shape to the sample beam profile (1202) is purposeful toindicate the possibility of such a profile. The origin of such a profilemay arise from two separate phenomenon. First, the laser may produce anelliptical beam due to different divergence angles along orthogonaldimensions. For various reasons such as eye pupil dimensions, the FOV onthe retina, cost, compactness, etc., it may be desirable to keep such anelliptical profile. Second, even when provided with a circular samplebeam profile, gratings are typically tilted resulting in elliptical beamprofiles incident on the grating. Although drawn such that the majoraxis of the sample beam ellipse is orthogonal to the grating grooves,such a design is not required, and it may be desirable to have the majoraxis parallel to the grooves (1206).

Compact Spectrometer

In FIG. 11(b), the beamsplitter (1100) is imaged (via two lenses, 1110and 1112) to the grating (1108). This allowed for an optional entranceslit or spatial filter (1106). It might not be necessary to include sucha slit or filter. In such an embodiment, spectrometer designs asdepicted in FIGS. 13(a) and 13(b) could be employed. Here, thediffraction grating (reflective in FIG. 13(a), indicated by referencenumeral 1300, and transmissive in FIG. 13(b), indicated by referencenumeral 1302) is placed near the beamsplitter (1304). Similar to in theembodiment in FIG. 11(b), an optical element (1306) and 2D sensor (1308)would complete the imaging spectrometer. This eliminates the need forthe two imaging lenses (1110 and 1112) in FIG. 11(b), but does not allowfor the optional spatial filter (1106). With a careful optical design,strong back reflections from optical components could be eliminatedresulting in an attractive design. Not only are the designs in FIGS.13(a) and 13(b) much more compact than the design in FIG. 11(b), butthey are also less expensive. Such compact spectrometers would becompatible with both double-pass and high-efficiency interferometricsetups as described above in Sections “Use of Pulse Tuned Source in aDouble-pass Interferometric System” and “Use of Pulse Tuned Source witha High-efficiency Interferometer”

In the situation where the grating must be separated away from thebeamsplitter (such as might be the case in FIG. 13(a) and FIG. 13(b)),the grating and the beamsplitter need not be both placed in a Fourierplane of the camera. The spectral and spatial resolution may becompromised depending on the setup. In one embodiment, the beamsplitteris placed in a Fourier plane of the camera to maintain high spatialresolution. This ensures the camera is in a plane conjugate to thesample. Furthermore, because the incident beam on the grating iscollimated along the dimension in which the spectrum is dispersed, afterthe grating, the collimated beams of different wavelengths would stillbe focused on the camera plane. The spectral resolution may only beslightly compromised due to different incident angles of differentwavelengths (i.e. the setup is non-telecentric along the spatialdimension). In another embodiment, the grating is in a Fourier plane ofthe camera to achieve high spectral resolution; however the sample maybe defocused along the orthogonal (spatial) dimension. To avoid a lossof resolution in this dimension due to defocus blurring, computationalrefocusing in this dimension could bring the sample back into focus.Using techniques such as holoscopy (see for example, US Publication No.2014/0028974) along the one spatial dimension, optimal resolution couldbe recovered. In yet another embodiment, two cylindrical lenses ofdifferent focal lengths could be employed such that both the spatial andspectral dimensions are optimally focused on the camera plane withoutloss of spatial and spectral resolution. Implementation of variousoptical elements, for example, focusing grating or grism, may furtherreduce the size of the spectrometer and would be recognized by thoseskilled in the art upon reading the teachings herein.

Self-interference Interferometry

Self-interference or self-referenced interferometry is a configurationwhere there may be additional synergies with special applications of asource of the present application. Self-referenced interferometry is adepth resolved interferometric technique, very similar to OCT, whereinthe ‘reference light’ originates in, or on the sample itself, ratherthan in an explicitly defined reference arm which is part of the imagingsystem. For example, a self-interface interferometer can be the systemof FIG. 8 without the reference arm. The reconstructed depth can beoptimally thin if the reference surface is in contact with, or in aportion of the sample of interest. Self-interference interferometry hasthe additional advantage where axial motion of the imaged object tendsto move all interfering scatterers in unison, and therefore does notcause axial motion induced phase washout. In this case, the integrationand sweep time can be increased up to the point where lateral motion ofthe sample becomes a limiting factor. Longer integration times mayenable sensing by a slower rate detector array and enable delivery ofmore energy in a single acquisition. Because lateral alignment betweenthe interfering portions of the beam is inherent, self-interferenceinterferometry is also tolerant to spatially multimode lasers.

Although self-interference interferometry is compatible with previoussystem configurations such as the double-pass interferometer,high-efficiency interferometer, 2-D spectrometer, and compactspectrometer (with a simple blocking of the explicit reference path),other configurations are also uniquely possible with this modality.Applications requiring small imaging depths require less spectralresolution at the detector. In cases where the imaging depth required isvery thin (one of which is self-interference interferometry), the pulsetuned diode may be used directly as a swept source in a traditional timeresolved detecting scenario. Instead of a 2D spectrometer as describedin the hybrid SS/SD-OCT solutions above, a fast array detector (eitherlinear or 2D) is used in this case to resolve the swept spectrum as atemporal series, with relatively poor spectral resolution. Thisconfiguration simplifies the optical configuration of the system. Itdoes, however require relatively fast electronics, which may befrequently idle, as the source requires a low duty cycle during themeasurement period.

Sources which are spatially multimode will not interfere with highcontrast if an attempt is made to interfere spatially distinct regionsof the field with each other. Spatially multimode sources are moredifficult to manage than spatially single mode sources when illuminatingan area. Although the multiple modes may illuminate the sample and aretransmitted to the detector, the difficulty arises because the referencearm must accurately align the same spatial modes to overlap on thedetector. A special case where alignment is inherently maintained isself-interference interferometry. Spatially multimode sources whichtolerate very high current (such as SPL PL85 OSRAM Opto Semiconductors,GmbH) are designed for laser range finders and are available at avariety of wavelengths in the infrared wavelength region. These sourcescan additionally be low cost because the pulsed driver is availableintegrated into the packaging and require only a low voltage DC voltagedrive. Laser range finders have traditionally operated around 850, 920,and 1064 nm. For low cost applications in the near future, the largeconsumer application of the diode laser will define the availablewavelengths.

In the above description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofthe specification. It should be apparent, however, that the subjectmatter of the present application can be practiced without thesespecific details. It should be understood that the reference in thespecification to “one embodiment”, “some embodiments”, or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin one or more embodiments of the description. The appearances of thephrase “in one embodiment” or “in some embodiments” in various places inthe specification are not necessarily all referring to the sameembodiment(s).

The foregoing description of the embodiments of the present subjectmatter has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit the presentembodiment of subject matter to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. It is intended that the scope of the present embodiment ofsubject matter be limited not by this detailed description, but ratherby the claims of this application. As will be understood by thosefamiliar with the art, the present subject matter may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof.

1. An optical system for imaging a sample comprising: a diode laserhaving a junction for providing a beam of radiation, said diode laserhaving a first spectral output bandwidth when driven under constantcurrent conditions; a driver circuit to apply a pulse of drive currentto the diode laser, said pulse having a current amplitude that varies intime causing a variation in the output wavelength of the diode laserduring the pulse to produce a second spectral output bandwidth that isat least two times larger than the first spectral output bandwidth underconstant current conditions and wherein the rate of change of thecurrent amplitude of the pulse is slow enough such that the differencebetween the actual temperature of the junction and the equilibriumtemperature of the junction is minimized during the pulse; optics fordirecting the beam of radiation to a sample to be imaged; a detector forproducing an electrical signal based on photon energy returning from thesample; and a processor to produce an image based the electrical signal.2. The system of claim 1, wherein the diode laser is a vertical cavitysurface emitting laser (VCSEL).
 3. The system of claim 1, wherein thedetector has an array of photosensitive elements.
 4. The system of claim1, wherein the detector is a camera.
 5. The system of claim 1, whereinsaid system is an optical coherence tomography system.
 6. The system ofclaim 1, wherein said system is a point scanning swept-source opticalcoherence tomography (SS-OCT) system.